Numerous methods have been proposed heretofore as methods modifying the surface of metal substrates by forming a silica coating on the surface of metal substrates. For example, Japanese Unexamined Patent Application Publication (hereinafter referred to as “JP Kokai”) No. S62-88327 (Patent Reference 1) teaches a method for forming a silica film by coating a toluene solution of a polymer derived from an organocyclosilazane onto a silicon wafer, drying the solution at room temperature, and then baking the polymer at high temperature in the presence of oxygen or water vapor. JP Kokai No. H9-157594 (Patent Reference 2) teaches a method for forming a silica coating by coating a xylene solution of perhydropolysilazane onto a stainless steel plate, or a metal, glass, ceramics and so on, and then heating the perhydropolysilazane at 450° C. in atmospheric air.
JP Kokai No. H10-155834 (Patent Reference 3) teaches a method for forming a silica coating by coating a xylene solution of perhydropolysilazane onto a silicon wafer, exposing the coated perhydropolysilazane to an amine compound and water vapor, and then baking the exposed perhydropolysilazane. JP Kokai No. 2004-155834 (Patent Reference 4) teaches a method for forming a silica layer by coating the surface of a metal, glass, plastics and so on with a xylene solution of perhydropolysilazane containing an amine catalyst and leaving to stand at room temperature.
But these methods relate to forming a silica layer on the surface of a metal, glass, plastics and so on to modify it to a heat-resistant, abrasion resistant and corrosion resistant surface. The surface of the substrate obtained by these methods does not have hydrophilic properties and compatibility with a human body. The substrate obtained by these methods is not satisfactory as a substrate used in a contact with a human body, specifically a living body.
JP Kokai No. H7-82528 (Patent Reference 5) teaches a coating solution comprising a mixture of polysilazane and an alcohol (for example, ethanol, ethylenegylcol, propyleneglycol), but the formed silica layer has insufficient hydrophilicity, small hardness, and inferior abrasion resistance.
JP Kokai No. 2003-327908 (Patent Reference 6) teaches an undiluted solution of a hydrophilicity-maintaining agent or hydrophilicity-accelerator comprising an aqueous solution of an anionic surface active agent, an amphoteric surface active agent, and a non-ionic surface active agent. According to this patent reference, overcoating the polysilazane-containing coating with it maintains and promotes the hydrophilicity. But since the anionic surface active agent, amphoteric surface active agent, and non-ionic surface active agent adhere to the surface of the silica layer, such silica layer is not satisfactory as a substrate used in contact with a human body, and has insufficient durability of hydrophilicity.
Whereas there is a trial to modify a metal substrate surface to a hydrophilic surface, but it is difficult to bond such hydrophilic polymer to a metal surface directly. No substrate with surface characteristics such as hydrophilicity, chemical resistance, and compatibility with a human body is provided.    [Patent Reference 1] JP Kokai No. S62-88327    [Patent Reference 2] JP Kokai No. H9-157594    [Patent Reference 3] JP Kokai No. H10-194753    [Patent Reference 4] JP Kokai No. 2004-155834    [Patent Reference 5] JP Kokai No. H7-82528    [Patent Reference 6] JP Kokai No. 2003-327908